Liquid crystal display devices utilized for monitors of cell-phones and personal computers, televisions, etc., are display devices having such a structure that liquid crystal molecules are sandwiched between liquid crystal alignment films each formed on a substrate, and utilizing a response of the liquid crystal molecules aligned in a certain direction by the liquid crystal alignment films to the voltage.
The liquid crystal alignment film is formed from a liquid crystal aligning agent, and determines the alignment of liquid crystal molecules and the pretilt angle of liquid crystal molecules and has substantial influences over electrical characteristics of the display devices, and thereby has a major role in the liquid crystal display device.
Heretofore, liquid crystal alignment films to improve characteristics of display devices have been developed. For example, it has been proposed to provide a liquid crystal alignment film capable of providing a display device having a high voltage holding ratio and a low residual charge by use of varnish containing a polyamic acid obtainable by condensing a diamine containing bis(aminomethyl)-bicyclo[2,2,1]heptane as the main component and a tetracarboxylic dianhydride, a partially imidated polyamic acid and a polyimide (for example, Patent Document 1). Further, a liquid crystal alignment film having improved alignment of liquid crystal by using a specific aliphatic diamine and an aromatic tetracarboxylic dianhydride has been proposed (for example, Patent Document 2).
However, in a case where an alicyclic or aliphatic diamine is used, it may form a salt with the corresponding amic acid, and if the salt has a low solubility, the reaction may terminate in some cases. Further, even if the salt has a high solubility, it is required to continue stirring until the formed salt disappears, and the reaction will take a long time as compared with a usual case. Accordingly, a process for producing a polyamic acid ester employing a silylamide type sililation reagent and a polyimide obtainable from the polyamic acid ester has been proposed (for example, Patent Document 3).
On the other hand, an aromatic diamine has good polymerization reactivity, but depending upon the type of the acid dianhydride to be reacted, the obtained polyamic acid may be non-uniformalized or may undergo gelation during storage at low temperature in some cases.
Since the productivity and the stability of the polyamic acids have substantial influences over the productivity and the stability of a liquid crystal aligning agent in some cases, the liquid crystal aligning agent has been desired to be capable of improving characteristics (particularly electrical characteristics) of the liquid crystal alignment film, in addition to be easily produced and have favorable storage stability.
Patent Document 1: JP-A-10-7906
Patent Document 2: JP-A-8-248424
Patent Document 3: JP-A-2002-322275